Search for Ultradense Nuclei in Relativistic Collisions of Ar on Pb

In 2.5×${10}^8$ interactions of 1.1- to 1.6-GeV/nucleon ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ ions with a Pb target we saw no tracks of products with $Z>\mathrm{}20\mathrm{}$ ejected in the beam direction, using a Lexan detector stack. The upper limit on the formation cross section is about 50 nb for products with $26\lesssim Z\lesssim 40$ and increases with $Z$ to about 1.5 μ b for $Z=100\mathrm{}$.